This invention relates to a fastener element such as screws, bolts, or nuts, and more particularly to the improvement in the contour of a bolt head or nut.
Generally, upon designing the bolt head, various items such as the transmission efficiency of torque, the possibility of the bolt head being damaged, the fitting of a driving tool to the bolt head, the so-called "cam-out" of the driving tool, the ease of manufacture, etc. should be taken into consideration.
Conventionally, a screw is known which is formed with a slotted head or cross recessed head. This type of screw has the drawbacks that the screw is limited in transmission efficiency of torque and that the inner wall of the slot or recess is likely to be worn or damaged.
In the customary bolt or nut including a hexagonal head or body, a so-called "driving angle," which is defined by a radial line passing a force-applied point with a tangential line to the driver contour at the force-applied point, is large, i.e., denotes 60.degree.. For this reason, the fastener driver is apt to slip on the outer wall of the bolt head when driving the bolt, so that torque loss is increased to cause the angular portions to be worn. As the result, said angular portions are likely to be rendered circular. In addition, a raw material of the bolt head has a circle-shaped cross section and should be subjected to trimming into a hexagonal cross section when worked, which unavoidably results in a complicated manufacturing process and increased manufacturing cost.
Further, U.S. Pat. No. 1,865,346 or French Patent No. 1,398,771 discloses a nut or bolt head formed at its outer periphery with grooves whose cross sections each assume a semicircular configuration. In this type of fastener element, a junction between one groove and the adjacent lobe formed between said one groove and another groove adjacent to the lobe is made angular, i.e., acute. Difficulties are therefore encountered in manufacturing the contour of the fastener element with high precision, and in addition said junction is very likely to be damaged when the fastener element is used. Further, though the fastener elements disclosed in said Patents might have a driving angle of 0.degree., any theoretical analysis for making the driving angle 0.degree. is not made. For this reason, when an attempt is made to form said acute junction into a curved configuration, i.e., a smooth configuration, there is a fear that the driving angle becomes more than 0.degree..